In the refining of lignocellulose-containing fibres by, for example, a disc refiner or a conical refiner at a low consistency of about 3 to 4%, the structure of the fibre wall is loosened, and fibrils or so-called fines are detached from the surface of the fibre. The formed fines and flexible fibres have an advantageous effect on the properties of most paper grades. In the refining of pulp fibres, however, the aim is to retain the length and strength of the fibres. In post-refining of mechanical pulp, the aim is partial fibrillation of the fibres by making the thick fibre wall thinner by refining, for detaching fibrils from the surface of the fibre.
Lignocellulose-containing fibres can also be disintegrated into smaller parts by detaching fibrils which act as components in the fibre walls, wherein the particles obtained become significantly smaller in size. The properties of so-called nanofibrillar cellulose thus obtained differ significantly from the properties of normal pulp. It is also possible to use nanofibrillar cellulose as an additive in papermaking and to increase the internal bond strength (interlaminar strength) and tensile strength of the paper product, as well as to increase the tightness of the paper. Nanofibrillar cellulose also differs from pulp in its appearance, because it is gel-like material in which the fibrils are present in water dispersion. Because of the properties of nanofibrillar cellulose, it has become a desired raw material, and products containing it would have several uses in industry, for example as an additive in various compositions.
Nanofibrillar cellulose can be isolated as such directly from the fermentation process of some bacteria (including Acetobacter xylinus). However, in view of large-scale production of nanofibril cellulose, the most promising potential raw material is raw material derived from plants and containing cellulose fibres, particularly wood and fibrous pulp made from it. The production of nanofibrillar cellulose from pulp requires the decomposition of the fibres further to the scale of fibrils. In processing, a cellulose fibre suspension is run several times through a homogenization step that generates high shear forces on the material. This can be achieved by guiding the suspension under high pressure repeatedly through a narrow gap where it achieves a high speed. It is also possible to use refiner discs, between which the fibre suspension is introduced several times.
International application PCT/FI2012/051116 (publication WO 2013/072559) shows a method where fibre material is introduced through several counter-rotating rotors in such a way that the material is repeatedly subjected to shear and impact forces by the effect of the different counter-rotating rotors while it flows outwards radially with respect to the rotors. Fibre material is made to nanofibrillar cellulose by feeding it at low consistency (1.5%-4.5%) through the rotors. The cellulose fibres used in this method as starting material are chemically modified so that the cellulose molecules have functional side groups which cause the weakening of the internal bonds in the cellulose fibre to facilitate the separation of fibrils. Catalytic oxidation and carboxymethylation are known chemical modification methods.
Conventionally the pulp is disintegrated to nanofibrillar cellulose at low consistency to guarantee good efficiency. This results in nanofibrillar cellulose in form of aqueous gel which has about the same nanofibril concentration as expressed in wt-%, that is, the nanofibrillar cellulose contains a great amount of water. Dewatering of nanofibrillar cellulose gels to increase the dry matter content has proved difficult. On the other hand, the pulp cannot be disintegrated to nanofibrillar cellulose at higher consistencies because the formation of fibrils remains poor and characteristic gel with high zero shear viscosity is not obtained. Thus, the production of large volumes of nanofibrillar cellulose is uneconomical because of the low production consistency.